1. Field of the Invention
The invention relates to a fluid pressure acutator that is specifically designed to operate in an environment involving fluids that attack metals either corrosively or by inducing stress corrosion cracking. Such environment is commonly found in actuators for oil field equipment.
2. Description of the Prior Art
Fluid pressure actuators are employed in a large variety of equipment for pipelines, subterranean wells, and the like. Such actuators are normally fluid pressure actuated to move a valve member to an open position and move the valve to a closed position by a highly compressed bias member. It is vitally important that such bias member retain all of its designed compressive force and that it not deteriorate or break under the severe environmental conditions to which such actuators are commonly subjected.
As is well known to those skilled in the art, such equipment is subjected to the combined hostile environment of H.sub.2 S, CO.sub.2 and/or various chlorides. The most effective and economical metals to employ for high compression springs for such actuators are various carbon steels, which are subject to relatively rapid failure either through corrosion, stress corrosion cracking or fatigue when required to continuously operate in the aforementioned hostile environment. The problem has been sufficiently serious that prior art attempts to solve it have involved the expensive use of more exotic and scarce metals for the springs or installing a control system that uses the pressurized corrosive fluid to pressurize non-corrosive and uncontaminated hydraulic oil. This necessarily means that highly corrosive well fluids can never be utilized as a source of control fluid pressure and that either a supply of non-corrosively treated oil is maintained at the well head, or expensive apparatus for separating the corrosive elements from the well fluids is maintained at the well head. Neither expedient is economically attractive.
Thus, there is a definite need for a fluid pressure actuator which provides protection for the highly compressed, carbon steels springs of such actuator from the hostile environment normally encountered in exposure to fluids produced from a subterranean well, without necessitating the utilization of a corrosive-free control fluid oil or the treatment of well fluids to remove the corrosive elements prior to application to the actuator or exotic spring materials that are immune to corrosive effects of well fluids.